Electronic devices containing organic semiconductor materials are known for use in devices such as organic light emitting diodes (OLEDs), organic photoresponsive devices (in particular organic photovoltaic devices and organic photosensors), organic transistors and memory array devices. Devices containing active organic materials offer benefits such as low weight, low power consumption and flexibility. Moreover, use of soluble organic materials allows use of solution processing in device manufacture, for example inkjet printing or spin-coating.
n-doping and p-doping of organic semiconductors has been investigated, for example n-doping using metals with a low ionisation potential such as Li, Na or Cs as disclosed in, for example, physica status solidi (a), 1/2013. In the case of n-doping, a problem arises in that a low ionisation potential of the n-dopant is needed for an electron to be transferred from the n-dopant to the lowest unoccupied molecular orbital (LUMO) of the organic semiconductor. This low ionisation potential renders the n-dopant susceptible to degradation in air.
Bao et al, “Use of a 1H-Benzoimidazole Derivative as an n-Type Dopant and To Enable Air-Stable Solution-Processed n-Channel Organic Thin-Film Transistors” J. Am. Chem.
Soc. 2010, 132, 8852-8853 discloses doping of [6,6]-phenyl C61 butyric acid methyl ester (PCBM) by mixing (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI) with PCBM and activating the N-DMBI by heating.
US 2014/070178 discloses an OLED having a cathode disposed on a substrate and an electron-transporting layer formed by thermal treatment of an electron-transporting material and N-DMBI. It is disclosed that a radical formed on thermal treatment of N-DMBI may be an n-dopant.
U.S. Pat. No. 8,920,944 discloses n-dopant precursors for doping organic semiconductive materials.
Naab et al, “Mechanistic Study on the Solution-Phase n-Doping of 1,3-Dimethyl-2-aryl-2,3-dihydro-1H-benzoimidazole Derivatives”, J. Am. Chem. Soc. 2013, 135, 15018-15025 discloses that n-doping may occur by a hydride transfer pathway or an electron transfer pathway.
Although N-DMBI has greater air stability than n-dopants such as alkali metals, it does still nevertheless undergo oxidation in the presence of molecular oxygen.
It is therefore an object of the invention to provide an n-doped organic semiconductor that may be formed from materials with higher air stability.